Hi Paul,
Am 24.11.2018 19:41, schrieb N1BUG:
Third test:
Moved dummy load to output of ferrite matching transformer with
ratio set for 1:1 impedance, so
PA> LPF> ScopeMatch sensor> long coax> 1:1 xfmr> dummy load. 15
minute RF carrier at 150W. At start, voltage and current on the
scope equal amplitude and in phase. After 15 minutes no change.
All right, so now the PA and the cable and the xfmr is not the problem,
as long as the xfmr sees Z=R=50 Ohm.
Did the xfmr stay completely cool?
Fourth test:
With coil and antenna, so
PA> LPF> ScopeMatch sensor> long coax> xfmr set to match initial
antenna R to 50 ohms> Loading coil> antenna. Started RF carrier at
150W, voltage and current on the scope equal amplitude and in phase.
After 45 seconds, voltage started to decrease, current started to
increase, and phase began shifting with voltage leading. After 3
minutes the phase stopped changing and remained with voltage leading
by a few degrees. After 6 minutes, voltage and current reached a
steady state with current much higher than initial. RF power had
increased significantly due to the lower Z load on Class-E PA. It
remained steady for the remaining 9 minutes of the test.
Did the xfmr become hot? It provides galvanic decoupling from the shack
earth and antenna earth, right?
Obviously your resonated antenna does not provide a Z=R=50 Ohm
impedance. Otherwise there would be no difference to the dummy load.
BTW, the output signal is usually a good sine wave i assume?
If the xfmr has just enough primary turns not to saturate at 50 Ohm, it
may thermally run away when connecting something higher than 50 Ohm
because then it starts to saturate.
It must be the core i think, because if the antenna detunes, the current
would decrease. But if the coil becomes hot, then it will!
The xfmr is switched between antenna earth and loading coil, in series,
and then the coil connected to the wire, right?
A long text is no problem.
73, Stefan
Fifth test:
Same as fourth test, but after phase became stable (3 minutes), I
retuned the coil (variometer) to bring phase back to zero. Voltage
continued to decrease and current continued to increase until the 6
minute point at which it reached steady state and did not change for
the remaining 9 minutes.
Sorry for the long text, but I wanted to describe exactly what I did
and the results.
I think:
PA OK
LPF OK
ScomeMatch OK
Coax OK
Transformer OK
Loading coil ???
Antenna ???
Ground system ???
Other nearby antennas, structures ???
73,
Paul
On 11/24/18 11:08 AM, DK7FC wrote:
Hello Paul,
Ooh it is a class-E PA? That explains much! They need a very accurate
SWR to reach the high efficiency. Initially i've been a friend of
class-E but what you see now is exactly the reason why i'm preferring
class-D now!
Generally: Avoid TO-220 cases for a real PA. Prefer TO-247 instead,
especially below 500 kHz.
Regarding the effect you're observing: I would guess it is a thermal
problem in a ferrite core, a capacitor or even the away-drifting
on-resistance / working point of the class-E PA.
73, Stefan
Am 24.11.2018 12:47, schrieb N1BUG:
Hi Markus,
That is very interesting. It changes about 25 % during the first 2-3
minutes, then it seems to settle down and not change any more. It
could be moisture somewhere, but any moisture here is solid ice or
frost now. I did not find any ice or frost in the transformer box or
the variometer. Could be insulators or something with the antenna
itself. I don't see any "fuzz" on the scomematch voltage trace so I
think (hope) nothing is arcing.
What worried me is this did not happen last winter so something has
changed. Everything accumulates some dirt here because of blowing
dust, smoke, etc. I wonder if a small amount of dirt on insulators
plus moisture can combine to make funny things happen.
Anyway I examined the little PA and it seems to have died due to
poor thermal interface between FET and heatsink. It's a physically
small FET and maybe was not screwed down tight enough with the
Sil-Pad interface, which was also a previously used one.
Normally I do not like to put drain voltage on the heatsink but as
an experiment for this little PA (which is totally an experiment
itself, but served me very well last winter) I will isolate the heat
sink from the chassis/PCB and mount the little FET directly to it.
The thermal resistance would be much lower! I think this is fine so
long as nothing shorts the heat sink to ground. In that case some
fuses die. ;-) This would add some pf of capacitance between drain
and ground but it would be in parallel with the quite large C of the
Class E tank, probably not much difference at 137 kHz!
Parts to repair the big PA should arrive Wednesday.
73,
Paul
On 11/23/18 4:29 PM, Markus Vester wrote:
Hi Paul,
sorry to read that. Have pity on the poor FETs!
You mentioned that the antenna resistance is gradually decreasing
(i.e. improving) during longer transmissions. I often see that
effect here, with the current rising by say 20 % during the first
few minutes. I've put it down to moisture or dew around the coil
and insulators (tiny little polycarbomnnate pencil tubes), which
evaporates as things warm up. It is more prominent during cold
damp weather, and much more so with the very high antenna
impedance at VLF than at LF.
Good luck, Markus
-----Ursprüngliche Mitteilung----- Von: N1BUG<[email protected]>
An: [email protected]<[email protected]>
Verschickt: Fr, 23. Nov. 2018 22:12 Betreff: LF: No 2200m TX
tonight
No transmissions from me this night. The little amplifier has
died. I think it may be related to this resistance change in the
antenna which is getting worse and worse.
I'm going back to MF for this night, sorry!
73, Paul
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